Metallic foams are a class of materials with very low densities and novel mechanical, thermal, electrical, and acoustic properties. In comparison to conventional solids and polymer foams, metal foams are light weight, recyclable, and non-toxic. Particularly, metal foams offer high specific stiffness, high strength, enhanced energy absorption, sound and vibration dampening, and tolerance to high temperatures. Furthermore, by altering the size and shape of the cells in metal foams, mechanical properties of the foam can be engineered to meet the demands of a wide range of applications.
Various methods are presently known in the art for preparing metallic foams. According to one method, metal powders are compacted together with suitable blowing agents, and the compressed bodies are heated above the solidus temperature of the metal matrix and the decomposition temperature of the blowing agent to generate gas evolution within the metal. Such “self-expanding foams” can also be prepared by stirring the blowing agents directly into metal melts. Metallic foams can also be prepared as “blown foams” by dissolving or injecting blowing gases into metal melts. Metallic foams can also be prepared by methods wherein gasses or gas-forming chemicals are not used. For example, metal melts can be caused to infiltrate porous bodies which are later removed after solidification of the melt, leaving pores within the solidified material.
Metallic foams have been shown to experience fatigue degradation in response to both tension and compression. Plastic deformation under cyclic loading occurs preferentially within deformation bands, until the densification strain has been reached. The bands generally form at large cells in the ensemble, mainly because known processes for producing these materials do not facilitate formation in a uniform manner. Such large cells develop plastically buckled membranes that experience large strains upon further cycling and will lead to cracking and rapid cyclic straining. As a result, the performance of existing foams has not been promising due to strong variations in their cell structure (see Y. Sugimura, J. Meyer, M. Y. He, H. Bart-Smith, J. Grenstedt, & A. G. Evans, “On the Mechanical Performance of Closed Cell Al Alloy Foams”, Acta Materialia, 45(12), pp. 5245-5259).
In the production of closed cell metallic foams, one obstacle is the inability to finely control cell size, shape, and distribution. This makes it difficult to create a consistently reproducible material where the properties are known with predictable failure. One method for creating a uniform closed cell metallic foam is to use prefabricated spheres of a known size distribution and join them together into a solid form, such as through sintering of the spheres, thereby forming a closed cell hollow sphere foam (HSF).
Hollow metal spheres, such as those available from Fraunhofer Institute for Manufacturing and Advanced Materials (Dresden, Germany), can be prepared by coating expanded plastic spheres (e.g., polystyrene) with a powdered metal suspension. The spheres are then placed into a mold and are heated to pyrolize the polystyrene and powder binder, and to sinter the metal powder into a dense, solid shell. Metal foams previously prepared through sintering of such hollow metal spheres are plagued by low strength. Foams prepared by sintering metal spheres made of stainless steel, when under compression, have been shown to undergo densification at a stress of approximately 2 to 7 MPa, reaching a strain of over 60%.
Accordingly, it is desirable to have metallic foams wherein cell size, shape, and distribution are controllable, and wherein high strength is exhibited. Such goals are achieved by the composite metal foams of the present invention and the methods of preparation thereof.